1. Field of the Invention
This invention relates generally to dado blades used with power saws, specifically to a magnetic dado shim for spacing apart dado blades.
2. Prior Art
A dado is a groove on a board for receiving the edge of another board, such as for joining perpendicular pieces of a cabinet, drawer, etc. A dado is cut with a set of dado blades, which typically includes a pair of circular blades spaced apart by one or more two-tooth blades, or four-tooth blades. Typical dado blades have standard 5/8" or 1-1/8" arbor holes for mounting on the drive sham or arbor of a power saw, and are tightened thereon by a nut and a washer. The blades are of a standard thickness, so that a suitable number of blades can be stacked together for cutting a dado for receiving a board of another standard thickness. E.g., two 1/8" thick circular blades can be spaced by two 1/8" thick two-tooth blades to make a 1/2" dado set for cutting a 1/2" dado for a 1/2" thick board.
Although boards are available in standard thicknesses, the actual thickness of any particular board can vary somewhat from its intended size due to poor manufacturing, shrinkage, expansion, etc. Therefore a dado cut to a standard width would often be slightly too narrow or too wide for snugly receiving a board.
Various adjustable dado sets have been proposed for providing an adjustable cutting width. U.S. Pat. No. 5,368,079 to Benway (1994) shows a dado set with a special blade having a non-standard, threaded hole for receiving a threaded collar. Turning the collar adjusts the spacing of the special blade from other blades. U.S. Pat. Nos. 4,589,458 to McCord, Jr. (1986) and 5,309,962 to McCord, Jr. et al. (1994) each shows an adjustable dado set with special blades having non-standard holes for receiving a pair of opposite cams. Turning the cams adjusts the spacing between the blades. However, the Benway and McCord devices both require special blades specifically designed for fitting their adjusting mechanisms, so that they cannot use conventional blades. Therefore, obtaining replacement blades for these devices is relatively difficult and expensive.
U.S. Pat. No. 5,316,061 to Lee (1994) shows plastic dado shims for positioning between and spacing apart dado blades. The shims are circular, and have an arbor hole and a radial slot for being installed on an arbor without removing the blades. The shims are provided in various thicknesses, so that one or a combination of several different shims can be used for minutely adjusting the spacing of a set of blades. Other dado shims commonly known in the carpentry trade are simple discs each with an arbor hole sized for closely fitting on an arbor, and are made of either paper or metal.
Unfortunately prior art dado shims are very difficult to install, because they frequently fall into and get caught in the relatively coarse thread of the arbor, i.e., they become eccentrically positioned on the arbor. As a result, the edges of their holes are cut or otherwise damaged when the blades are tightened together. Further, the crumpled shims cause the blades to be improperly spaced. When the shims fall into the arbor's thread, they hinder the installation as well as the removal of the blades. This problem is compounded several folds when several shims are used, because a properly placed shim would easily fall into the thread when the user is busy installing additional shims or blades. Therefore, ensuring the proper installation of most any prior art dado shim is a very frustrating and time consuming exercise. Furthermore, prior art paper and metal dado shims can easily become bent or otherwise damaged in harsh shop environments. Once damaged, they usually cannot be adequately flattened or repaired.
Still another problem is that blades stacked with nothing therebetween, or with metal shims therebetween, are prone to metal-to-metal vibration. This causes a chattering or whistling noise, which can become very annoying when the saw is used for prolonged periods of time. Also, when a user tries to measure the total width of a set of dado blades mounted on an arbor, but before they are tightened, the blades will tend to fall to one side of the arbor's coarse thread, so that they will not sit straight or parallel to each other. Therefore, accurate measurement of the set cannot be easily made.